Telephone, cable television (CATV) and other communication and control cables are often direct buried or placed in underground duct structures. For this type of cable placement, the most significant cause of cable outages is caused by dig ups by contractors. In an effort to minimize inadvertent dig ups "call before you dig" programs are heavily promoted. The operating company must then be able to quickly and accurately locate and mark the buried cable.
Methods have been developed and are in commercial use which place a locating tone on the cable sheath via the armour or shield and a special receiver with magnetic field detecting coils is used to sense the tone current travelling along the cable. The strength of the received signal is directly proportional to the magnitude of the tone current in the cable sheath directly below the receiver.
The transmission circuit for the tone signals is formed by a metal armour or shield of the cable, insulated by a plastic cable jacket from earth, which forms the return conductor. The circuit is basically a form of coaxial transmission path with the cable shield or armour forming the inner conductor, the plastic cable jacket the insulator and the surrounding earth forming the outer conductor.
As with any transmission line with resistive elements, the line is lossy, and the loss increases with increasing frequency. In an attempt to reach longer and longer distances, the tone frequency is selected as low as is practical and the input current and power level set as high as is reasonable and safe. Fibre optic cables are often installed in long lengths of up to 75 miles (120 km) or more. Even with the use of low frequencies and high power the signal is often weak or unusable at these long distances.
The situation may be further complicated by a requirement to transmit high frequency tones, which are often used for inductively coupled short distance location applications.
This application relates to the enhancement of the long distance low frequency tones. In preferred embodiments this will not impede the use of high frequency tones for inductively coupled short distance applications.